Vehicle hydraulic braking system with an active simulator

ABSTRACT

A hydraulic braking apparatus for a motor vehicle having a service braking system (A) fed by a central hydraulic unit ( 3 ) and an emergency braking system (B). A simulator (M) resists a forward motion of an actuator member (D,  16 ) for a master cylinder ( 17 ) to define a reaction corresponding to the progress of a braking operation. Admission solenoid values ( 9   a   -9   d ) and exhaust solenoid values ( 14   a-   14   d ) connected to the wheel brakes ( 2   a-   2   d ) modulate pressurized fluid supplied by a central hydraulic unit ( 3 ) in effecting the braking operation. Sensors ( 8, 13   a-   13   d,    24, 29 ) in the motor vehicle detect various braking parameters that are communicated to a computer (C) to control the solenoid valves ( 9   a-   9   d;    14   a-   14   d ) such that the braking operation is a function of the travel of the actuator member (D,  16 ).

BACKGROUND OF THE INVENTION

This invention relates to a hydraulic braking apparatus of the typecomprising, for the actuation of the wheel brakes:

-   -   a service braking system, supplied with a pressure brake fluid        by a central hydraulic unit, using an external energy source;    -   an emergency braking system, controlled by muscular energy;    -   a hand-control member, the forward travel of which actuates the        service braking system or, in the case of a failure of the        latter, the emergency braking system;    -   A master cylinder having at least one primary piston, the stroke        of which is controlled by the hand-control member;    -   at least one safety valve, capable either of separating the        master cylinder from the wheel brakes when the service braking        system operates properly or, should the service braking system        fail to operate correctly, of connecting the master cylinder        with at least one wheel brake;    -   a feeling simulator, intended to resist the forward motion of        the hand-control member with a reaction corresponding to the        progress of a braking operation, such simulator comprising a        cylinder in which a simulator piston may slide while being        subjected, in one direction, to a fluid pressure from the master        cylinder and, in the opposite direction, to a counterforce        dependent on the travel of the hand-control member;    -   pressure-fluid admission solenoid valves and exhaust solenoid        valves, connected to the wheel brakes;    -   sensors for the detection of various braking parameters, in        particular the travel of the hand-control member, and the        pressures at various spots of the apparatus;    -   and a computer, connected to the various sensors and capable of        controlling the solenoid valves so as to obtain the desired        pressures in the wheel brakes.

A braking apparatus of said type is known, for instance, from FR 2 772706 or from U.S. Pat. No. 5,544,948.

In such an apparatus, in the course of a trouble-free operation in theservice braking mode, the master cylinder is isolated and the fluid,contained in the master cylinder, cannot flow back to the wheel brakes.The hand-control member, e.g. a brake pedal or a hand-brake lever,retains a normal actuating travel, dependent on the exerted force,thanks to the feeling simulator, which comprises a cylinder connected tothe master cylinder for the fluid transfers.

The well-known apparatuses operate satisfactorily and, besides, theymake it possible to lay down a law of variation for the force to beapplied to the hand-control member as a function of the travel, whichmay give the driver a feeling like that he would get if the pressureinside the wheel brakes resulted directly from the pressure supplied bythe master cylinder, and from the muscular force exerted on the brakepedal.

Yet, in these apparatuses known per se, the law of variation concerningthe force to be applied to the hand-control member is somewhat fixed,and it cannot be altered in a simple and rapid manner.

Now then, on various grounds, more especially depending on the type ofthe motor vehicle concerned, it is most desirable that said law ofvariation should be alterable as simply and as rapidly as possible.

Besides, it is most desirable that the simulator should consume aslittle fluid as possible, so that the emergency braking, achieved withthe help of the master cylinder, may remain as efficient as possible.

BRIEF SUMMARY OF THE INVENTION

Therefore, the primary object of the present invention is to provide ahydraulic braking apparatus, which meets the various above-mentionedrequirements better still than currently and which, more particularly,makes it possible to alter, in an easy and rapid manner, the law ofvariation for the force to be applied to the hand-control member, as afunction of the travel.

Moreover, it is to be desired that the solution, as it is providedherein, be implemented in a comparatively simple and especially reliableway.

According to the invention, a hydraulic braking apparatus for a motorvehicle, of the above-defined kind, is characterised in that thecounterforce within the simulator results from the action, on a surfaceof the simulator piston, of a modulated pressure, which comes from thefluid pressure supplied by the central hydraulic unit and is controlledby the computer in accordance with a determined law, alterable ad lib,as a function of the pedal travel.

Any law of variation whatever, as regards the force exerted on thehand-control member as a function of the travel, may be programmed inthe computer, without having to modify the apparatus in another way.

Preferably, the surface of the simulator piston, which is under themodulated pressure, defines a variable-capacity chamber connected inparallel to an admission solenoid valve, for the pressure fluid suppliedby the central hydraulic unit, and to an exhaust solenoid valve,connected to the feed tank, the opening and closing of said solenoidvalves being controlled by the computer so that the pressure inside thesimulator chamber may follow the desired law.

In a preferred manner, the solenoid valves, connected to the simulatorchamber, are of the “on/off” type and the pressure drop between theinlets and the outlets of the solenoid valves may be linear dependentlyon the control current strength.

Advantageously, in the simulator, the counterforce is the resultant of aresilient force acting upon the simulator piston in the oppositedirection to the fluid pressure coming from the master cylinder, and ofa variable force resisting the resilient force, such variable forcebeing generated by the modulated pressure acting on a surface of thesimulator piston.

The resilient force may be produced by at least one resilient returnmeans. In a preferred manner, such resilient return means comprises anair spring.

The cylinder of the simulator may comprise two intercommunicatingcoaxial bores with different diameters, and a stepped piston including asmall-diameter portion sliding in a leak proof manner inside thesmall-diameter bore, and a greater-diameter portion sliding in a leakproof manner in the large-diameter bore, the end wall of thesmall-diameter bore comprising a port connected to the master cylinderfor the fluid pressure from the master cylinder to be applied to thesmall cross-section of the stepped piston, whereas an annular chamber isdefined between the transition wall of the bore and the largecross-section of the stepped piston, such annular chamber beingconnected in parallel to the respective admission and exhaust solenoidvalves.

The end wall of the simulator cylinder, which closes the largecross-section bore in the opposite direction to the small cross-sectionbore, may comprise an opening for the passage of a rod resting on thelarge cross-section of the stepped piston and exerting the resilientforce on said piston. Such rod may be integral with a pneumatic pistonsliding inside a pneumatic cylinder, for its part integral with thesimulator cylinder, such pneumatic cylinder being connected to anexternal air-pressure source, intended e.g. for a pneumatic suspension.

A nonreturn valve may be provided on a pressure-air line connected tothe pneumatic cylinder, such valve allowing the inflow of the pressureair into the cylinder and opposing its outflow.

A mechanical compression spring may be arranged inside the pneumaticcylinder to as to act upon the pneumatic piston in the same direction asthe air pressure.

In addition to the above-mentioned arrangements, the invention providesa number of arrangements as well, which will be more fully explained inthe following detailed description of an embodiment of the presentinvention, by way of example and by no means as a limitation, when takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a hydraulic braking apparatusaccording to the present invention;

FIG. 2 is a simplified diagram on a larger scale of the simulator and ofthe master cylinder;

FIG. 3 illustrates an example of a law of variation for the force to beexerted on the hand-control member as a function of the travel, and ofthe modulated pressure; and

FIG. 4 shows the variation of the air pressure inside the pneumaticcylinder as a function of the piston stroke.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a hydraulic braking apparatus 1 intended for a motorvehicle and devised to actuate the wheel brakes 2 a and 2 b for thefront wheels, and 2 c and 2 d for the rear wheels. In a conventionalmanner, each wheel brake comprises a cylinder in which a piston iscapable of moving under the action of a pressure fluid, so as to apply abrake pad or shoe against an element, either a disk or a drum,rotationally locked with the wheel to be braked.

The apparatus 1 comprises a service braking system A, supplied with apressure fluid by a central hydraulic unit 3, using an external energysource, and an emergency braking system B, controlled by muscularenergy.

The central hydraulic unit 3 comprises a pump 4 driven by a motor 4,e.g. an electric motor. The pump 4 delivers pressure fluid to a mainsupply line 6, on which a hydropneumatic accumulator 7 is mounted. Apressure sensor 8, which outputs an electrical quantity indicative ofthe pressure value in the line 6, is also fitted on said line. The inletside of the pump 4 is connected to a nonpressure fluid tank 9, alsocalled a feed tank.

The pressure fluid line 6 is connected in parallel, through solenoidvalves, 9 a, 9 b, 9 c and 9 d, to the respective wheel brakes 2 a-2 d.Such solenoid valves are two-position ones, i.e. they are either open orclosed, and they are pilot-controlled by means of a programmablecomputer, or microprocessor C. For a better and clearer understanding ofthe drawings, the electrical connections between the control coils ofthe solenoid valves and the computer C are represented by the beginningof a line only.

In the rest position, the valves 9 a-9 d are closed, as shown in FIG. 1.The outlet of a valve 9 a, 9 b is connected to a front wheel brake 2 a,2 b through a hydraulic separator, 10 a and 10 b respectively. Theoutlets of the valves 9 c and 9 d are directly connected to the rearwheel brakes 2 c, 2 d. A pressure-equalizing valve 11 is intercalatedbetween the outlets of the valves 9 a and 9 b, and anotherpressure-equalizing valve 12 is intercalated between the outlets of thevalves 9 c, 9 d as well.

A pressure sensor 13 a, 13 b, 13 c, 13 d is fitted on each supply linefor the brakes 2 a-2 d so as to output an electrical quantityrepresentative of the applied pressure. The outputs of these sensors 13a-13 d are connected by lines (not shown) to the computer C. The outputof the sensor 8 is also connected to C.

The exhaust solenoid valves 14 a, 14 b, 14 c and 14 d are connected inparallel to the admission solenoid valves 9 a-9 d, on the lines whichare connected to the inlets to the wheel brakes. Said valves 14 a-14 dare two-position ones, i.e. either an open or a closed position, andthey are connected to a line 15 returning the fluid to the feed tank 9.At rest, the valves 14 a-14 d are open, as shown in FIG. 1.

The exhaust valves 14 a-14 d are also pilot-controlled by the computerC, which comprises outputs connected to each coil controlling the valves14 a-14 d.

The apparatus comprises a hand-control member D, generally consisting ofa brake pedal 16 and a master cylinder 17, in which a primary piston 18and a secondary piston 19 may slide, both of them having the samecross-section S1. The pedal 16 is connected to the piston 18 by a rod20, linked to the pedal. Herein, a “forward travel or motion” designateda motion of the pedal 16 towards the master cylinder 17, which bringsabout a travel of the piston 18 towards the secondary piston 19 and theopposite end wall of the cylinder 17.

A primary chamber 21, filled with fluid, is defined between the piston18 and the piston 19, and a spring 21 a is disposed in said chamberbetween both pistons. A second chamber 22, filled with fluid too, isdefined between the piston 19 and the end wall of the master cylinder17, remote from the piston 18. A spring 22 a is arranged in the chamber22.

An electric contact 23, sensitive to the forward travel of the pedal 16,is provided in a conventional manner for the control of the “stop”lights. A terminal of this contact 23 is connected to a terminal of thecomputer C, which actuates the service braking system A in response tothe forward travel of the brake pedal 16. Besides, a stroke sensor 24for the pedal 16 is provided and it transmits a corresponding electricalsignal to another input terminal of the computer C. For instance, thesensor 24 may output data concerning the amplitude of the travel of thepedal 16 as well as data regarding the travel speed.

Both chambers 21, 22 of the master cylinder are connected to the feedtank 9 through a nonreturn valve (not shown), which enables the chambers21, 22 to be supplied with fluid but precludes a backflow.

The primary chamber 21 is connected with the wheel brake 2 b, by meansof a pipe 25 fitted with a safety or stop solenoid valve 26. Thesolenoid valve 26 is controlled by the computer C and it is of thetwo-position type, i.e. open or closed; it is in the open position whenthe apparatus is at rest.

The chamber 22 is connected to the wheel brake 2 a through a line 27 anda solenoid valve 28. A pressure sensor 29 is fitted on the line 27,between the master cylinder 17 and the solenoid valve 28. The sensor 29outputs an electrical signal, which is applied through a link (notshown) to an input of the computer C.

Besides, the braking apparatus 1 comprises a brake-actuation simulatorM, intended to resist the forward motion of the brake pedal 16 with areaction corresponding to the progress of a braking operation.

Such simulator M comprises a cylinder 30 (cf. FIGS. 1 and 2), in which asimulator piston 31 slides.

The cylinder 30 comprises two intercommunicating coaxial bores 30 a, 30b with different diameters. The smaller-diameter bore 30 a is bounded,in the opposite direction to the bore 30 b, by a wall 30 c in which acentral port 32 is provided. Such port 32 is connected through a pipe 33with one chamber of the master cylinder 17, in the present instance thesecondary chamber 22.

The piston 31 is a stepped piston including a small-diameter portion 31a with a cross-section S2, sliding in a leak proof manner inside thebore 30 a, and a greater-diameter portion 31 b sliding in a leak proofmanner in the bore 30 b. The portion 31 b is edged with a cylindricalskirt, the concave side of which is in the opposite direction to thebore 30 a.

An annular chamber 34, with a cross-section S3, is formed between theportion 31 b and the transition wall 30 d situated between the bores 30a and 30 b. Said annular chamber 34 surrounds the portion 31 a, and itscapacity varies as a function of the position of the piston 31 along theaxis of the cylinder 30. A port 35 is provided in the wall of thecylinder 30 and it opens into the bore 30 b, near the wall 30 d, whichis the end wall of the chamber 34.

The port 35 is connected in parallel (FIG. 1) to a pressure-fluidadmission solenoid valve 36 and to an exhaust solenoid valve 37. Thesolenoid valves 36 and 37 are of the “on/off” type, which means thatthey have two positions and that they are either open or closed.Preferably, the pressure drop between the inlets and the outlets of thesolenoid valves 36, 37 follows a linear variation, as a function of thecontrol current strength for these valves. The control coils of thevalves 36, 37 are connected by electrical lines 38 and 39 to twoterminals of the computer C. The inlet of the valve 36 is connected tothe pressure-fluid supply line 6 coming from the central hydraulic unit3. The outlet of the valve 37 is connected to the fluid-return line 15leading to the tank, or feed tank 9.

The computer C controls the valves 36 and 37 as a function of the travelof the pedal 16 so to obtain a modulated pressure Pehb, which is appliedwithin the annular chamber 34 and exerted on the annular surface S3 ofthe piston 31.

The end wall 30 e, closing the bore 30 b in the opposite direction tothe bore 30 a, comprises an opening 40 for the passage of a rod 41,which is coaxial with the cylinder 30 and bears on the piston 31. Therod 41 is integral with a pneumatic piston 42 (i.e. a piston subjectedto a gas pressure), disposed inside a cylinder 43 coaxial with thecylinder 30 and attached to the latter. Generally, the diameter of thepiston 42 is greater than the diameter of the portion 31 b of the piston31. These diameters are determined so as to achieve the desired forces,while taking into account the pressures involved. The rod 41 traversesthe end wall of the cylinder 43.

The chamber 44 of the cylinder 43, located on the rod 41 side, isconnected to the atmosphere through at least one port, not visible inthe drawings. In the same way, the chamber 45 in the bore 30 b, whichreceives the rod 41, is connected to the atmosphere through at least oneport, not visible on the drawings either.

In the opposite direction to the rod 41, the piston 42 has across-section S4 and it defines, inside the cylinder 43, a chamber 46with the same cross-section S4. Such chamber 46 is connected, via a port47 provided in the end wall remote from the cylinder 30, to a pipe 48,for its part connected to an external air pressure source 49. Moreparticularly, the source 49 may be a compressed-air source for apneumatic suspension. By way of a non-limiting example, thecompressed-air pressure, supplied by the pipe 48, may be in the order of10 bars.

A nonreturn valve 50 is provided on the pipe 48, near the port 47, so asto allow the inflow of the pressure air from the pipe 48 into thechamber 46 and to prevent an air flow in the reverse direction.

A compression spring 51 is arranged in the chamber 46, between thepiston 42 and the end wall of the chamber, for an action in the samedirection as the air pressure. Such spring 51 exerts on the piston 42but a negligible return force, compared with the forces generatedthrough the pressures.

Thus, in one direction, the piston 31 of the simulator is under thefluid pressure coming from the master cylinder 17 and exerted on thesmall portion 31 a and, in the opposite direction, it is subjected to acounterforce, which depends on the travel of the pedal 16. Suchcounterforce is equal to the difference between the resilient force,applied by the piston 42 and transmitted through the rod 41, and thevariable force, which is exerted by the modulated pressure Pehb on thecross-section S3 of the stepped piston 31.

The simulator M takes action when the service braking system operates ina trouble-free manner. Under those circumstances, the valves 28 and 26are closed so that the fluid within the chamber 21 is confined in aconstant volume; as a matter of fact, the pressure prevailing in saidchamber 21 is the same as that existing in the chamber 22, connected tothe pipe 33.

FIG. 2 is a simplified diagram, which makes it possible to lay downrelationships between the various quantities. The various parameters aredesignated as follows:

-   Frod=force exerted by the pedal 16 on the rod 20-   Pmc=pressure inside the master cylinder 17-   S1=cross-section of the master cylinder 17-   Xt=travel of the rod 20 and of the piston 18-   S2=cross-section of the portion 31 a-   Xsimu=travel of the piston 31-   S3=cross-section of the annular chamber 34-   Pehb=modulated pressure at the port 35-   S4=cross-section of the piston 42, on the chamber 46 side-   P0=initial pressure in the chamber 46-   V0=initial volume of the chamber 46-   h0=initial axial length of the chamber 46.    In the absence of fluid leaks:-   S1.Xt=S2.Xsimu, whence Xt=(S2/S1).Xsimu.

This being so, the mode of operation of the apparatus is as follows.

At rest, that is when the pedal 16 is not depressed, the variousconstitutive parts of the apparatus are in the positions illustrated inFIG. 1.

As soon as the pedal 16 is actuated, the contact 23 sends the computer Cdata indicating the beginning of a braking operation. The computer Ccauses the closure of the valves 26 and 28, thus separating the mastercylinder 17 from the brakes 2 a, 2 b of the front wheels. Besides, thecomputer C controls the solenoid valves 9 a-9 d and 14 a-14 d so as toinduce, in the wheel brakes 2 a-2 d, a pressure which is a function ofthe travel of the pedal 16, more particularly a function of the positionand the travel speed of the latter. Other factors, e.g. the detection ofa wheel locking may be taken into consideration by the computer C so asto act on the brake pressure.

Moreover, the computer C controls the valves 36, 37 in order to obtain,at the inlet 35, a modulated control pressure Pehb, which variesaccording to a predetermined law, as a function of the pedal travel.

The curve L1 represented in FIG. 3 is an example of a law of variationfor the pressure Pehb, the values of which are indicated by thebar-graduated scale along the Y-axis on the right-hand side, as afunction of the pedal travel expressed in millimeters along the X-axis.

When the piston 42 travels in the direction meaning an increased volumeof the chamber 46, it is under an air pressure, equal to that suppliedby the line 48. But, when the piston 42 moves in the direction resultingin a reduced capacity of the chamber 46, the valve 50 closes and the airvolume confined inside the chamber 46 undergoes a compression process,such a compression being generally considered as adiabatic, so that theair pressure rises in the chamber 46.

The force, exerted by the pressure Pehb on the cross-section S3 of thepiston 31, is subtracted from the force applied by the piston 42. Thepressure Pmc of the master cylinder, applied to the cross-section S2 ofthe portion 31 a, balances such difference. Said pressure Pmc, appliedto the piston 18 of the master cylinder, generates the reaction,resisting the forward travel of the pedal 16.

The pressure inside the chamber 46 being designated by Px, for an axiallength (h0−Xt) of this chamber, the expression may read:Prod=Pmc.S2=(Px.S4)−(Pehb.S3)and the various quantities may be inferred from the relationshipexisting between the pressure Px and the volume of the air mass confinedin the chamber 46.

The variation of the force Ft to be exerted on the rod 20 as a functionof the travel of such rod is illustrated by the curve G1 in FIG. 3, thevalues of the force Ft being indicated by the newton-graduated scalealong the Y-axis, on the left-hand side.

The law L1 controlling the pressure Pehb may be altered ad lib, througha programming of the computer C. It means that the curve G1 may bealtered ad lib too, without having to modify the equipment for all that.

At the beginning of the travel of the pedal 16, the force to be exertedon the rod 20 should not be too high, with the result that the pressurePehb is comparatively high in the case of short travels, so as to reducethe force to be exerted on the pedal 16.

The more the pedal 16 is depressed, the more the chamber 22 feeds fluidto the bore 30 a. The piston 31 travels towards the cylinder 43 whilepushing back the rod 41 and the piston 42. The volume of air, confinedinside the chamber 46, exerts an increasing pressure, which results in agreater force to be applied to the rod 20. The pressure Pehb isdecreasing from a certain value of the travel of the pedal 16 onwards,so that the resistance to the forward travel may be great enough towardsthe end of the travel.

At that time, the driver actually “feels” the level of the braking forceapplied by an external energy source, irrespective of his musculareffort.

FIG. 4 shows in a curve K1 the variation of the air pressure Px (in thechamber 46) expressed in bars along the Y-axis, as a function of thepiston 42 travel expressed in millimeters along the X-axis.

Should some trouble happen in the service braking system, the calculatorC would detect this failure, e.g. on the basis of too low a pressurevalue, output by the sensors 13 a-13 d, though the pedal 16 has moved.

As a consequence of it, the computer C keeps the valves 26, 28 in theiropen positions, with the result that pressure fluid, coming from themaster cylinder 17, may flow via two independent circuits towards thebrakes 2 a and 2 b, thus making it possible to carry out an emergencybraking operation.

Moreover, the fluid contained in the bore 30 a is driven under theaction of the piston 42 still under the air pressure, and a pressure,substantially higher than that prevailing inside the chamber 22 of themaster cylinder, is produced in the bore 30 a. Therefore, the piston 31is pushed back, to the left-hand side in FIG. 1, and it discharges somefluid into the line 27 supplying the brake 2 a, the effect of it beingthat the braking requirements are met still better.

As a matter of fact, in the presence of a failure in the service brakingsystem, the emergency braking system, based on the muscular energy, mustenable the driver to apply the brakes with a sufficient decelerationwhich is, for the time being, fixed at to 0.3 g, in response to adetermined force, e.g. 500 newtons (500 N), exerted on the brake pedal16. Owing to the fact that the fluid volume, coming from the bore 30 aof the simulator, is recovered for the emergency braking, such anemergency braking operation can be ensured even in the case of acomparatively heavy motor vehicle, e.g. in the order of 4,000 kg.

1. A hydraulic braking apparatus for actuating wheel brakes of a motorvehicle, comprising: a service braking system (A), supplied with apressure brake fluid by a central hydraulic unit (3), using an externalenergy source; an emergency braking system (B), controlled by muscularenergy; a hand-control member (D, 16), the forward travel of whichactuates the service braking system and in the case of a failure of theservice braking system, the emergency braking system; a master cylinder(17) having at least one primary piston (18), the stroke of which iscontrolled by the hand-control member (D, 16); at least one safety valve(26, 28) for separating the master cylinder (17) from the wheel brakes(2 a, 2 b) when the service braking system operates properly and shouldthe service braking system fail to operate correctly for connecting themaster cylinder with at least one wheel brake; a feeling simulator (M)for resisting the forward motion of the hand-control member (D, 16) witha reaction corresponding to the progress of a braking operation, saidsimulator comprising a cylinder (30) in which a simulator piston (31)slides while being subjected, in one direction, to a fluid pressurecoming from the master cylinder (17) and in the opposite direction, whensubjected to a counterforce corresponding to a predetermined travel ofsaid hand-control member (D, 16); pressure-fluid admission solenoidvalves (9 a-9 d) and exhaust solenoid valves (14 a-14 d), connected tothe wheel brakes; sensors (8, 13 a-13 d, 24, 29) for the detection ofvarious braking parameters including the travel of the hand-controlmember (D, 16), and the pressures at various spots of the apparatus; anda computer (C), connected to the various sensors and for controllingsaid solenoid valves to obtain desired pressures in said wheel brakes,characterised in that said counterforce within the simulator (M) resultsfrom the action on a surface (S3) of said simulator piston (31) of amodulated pressure (Pehb) received from the fluid pressure supplied bythe central hydraulic unit (3) and is controlled by the computer (C) inaccordance with a determined law (L1), alterable ad lib as a function ofthe travel of said hand-control member (D, 16); and in that said surface(S3) of said simulator piston receives the modulated pressure (Pehb)communicated to a variable-capacity chamber (34) within said cylinder(30) that is connected in parallel to an admission solenoid valve (36),for the pressure fluid supplied by the central hydraulic unit (3), andto an exhaust solenoid valve (37), connected to a feed tank (9), theopening and closing of said solenoid valves (36, 37) being controlled bythe computer (C) so that the pressure (Pehb) inside thevariable-capacity chamber (34) follows said determined law (L1) and inthat said solenoid valves (36, 37) are connected to saidvariable-capacity chamber (34) of said simulator (M) that is either onor off; and in that a pressure drop between an inlet and an outlet ofsaid solenoid valves (36, 37) is linear dependently on a control currentstrength.
 2. The apparatus according to claim 1, characterised in thatsaid cylinder (30) of said simulator (M) comprises a small diameter bore(30 a) separated from a coaxial large diameter bore (30 b) by atransition wall (30 d) and a simulator piston (31) including asmall-diameter portion (31 a) that slides in a leakproof manner insidesaid small-diameter bore (30 a), and a greater-diameter portion (31 b)that slides in a leakproof manner in said large diameter bore (30 b),said small diameter bore (30 a) having an end wall (30 c) with a port(32) connected to the master cylinder for the fluid pressure from themaster cylinder (17) that is applied to a small cross-section (S2) ofthe simulator piston (31), said transition wall (30 d), large diameterbore (30 b) and greater-diameter portion (31 b) of said piston definingan annular chamber (34) that is connected in parallel to said respectiveadmission and exhaust solenoid valves (36, 37).
 3. The apparatusaccording to claim 2, characterised in that, in the simulator (M), acounterforce is the resultant of a resilient force acting upon saidsimulator piston (31) in an opposite direction to fluid pressure comingfrom the master cylinder (17), and of a variable force resisting theresilient force, said variable force being generated by a modulatedpressure (Pehb) acting on a surface (S3) of said simulator piston
 31. 4.The apparatus according to claim 3, characterised in that a resilientforce is produced by at least one resilient return means.
 5. Theapparatus according to claim 4, characterised in that said resilientreturn means comprises an air spring (42, 43).
 6. The apparatusaccording to claim 3, characterised in that an end wall (30 e) of thesimulator cylinder (30), which closes the large diameter bore (30 b) inan opposite direction to said small diameter bore (30 a) has an opening(40) for the passage of a rod (41) that engages a large cross-section(31 b) of the simulator piston (31) to exert a resilient force on saidpiston (31).
 7. The apparatus according to claim 6, characterised inthat said rod (41) may be integral with a pneumatic piston (42) thatslides inside a pneumatic cylinder (43) that is an integral part of thesimulator cylinder (30), said pneumatic cylinder (43) being connected toan external air-pressure source (49) for pneumatic suspension.
 8. Theapparatus according to claim 7, further characterised by a nonreturnvalve (50) that is provided on a pressure-air line (48) that isconnected to the pneumatic cylinder (43), said valve (50) allowing theinflow of pressure air into the pneumatic cylinder (43) while opposingoutflow of air.
 9. The apparatus according to claim 8, furthercharacterised by a mechanical compression spring (51) that is arrangedinside the pneumatic cylinder (43) to act upon the pneumatic piston (42)in a same direction as pressurized air.